Alkali doped polyacetylenes

Alkali-doped polyacetylene is extremely air-sensitive and deteriorates much more quickly than oxidatively doped polyacetylene. But, whereas the latter degrades rapidly upon heating as a result of polymer-dopant reactions that alter the chain, alkali-doped polyacetylene is surprisingly stable thermally, up to 200°C. This can be exploited to anneal cis-rich doped samples, which leads to a considerable decrease of disorder and evolution towards the trans lattice [88]. Especially in K-doped samples, this leads to a strong conductivity increase. 

The first experiments were carried out on films of solid CeO) 100 to 1000 A thick, exposed to alkali metal vapors.[Ha91a ] It was observed that the conductivity increased by more than seven orders of magnitude, to 500 (fl cm) . This is still a much lower conductivity than any metallic element, but on the order of such organic systems as doped polyacetylene. Haddon et al. made the plausible suggestion, subsequently proved, that the smaller alkali ions were intercalated into the voids between the much larger fullerenes, and donated their charge to the unoccupied fullerene molecular orbital. The ratio of alkali atoms to fuUerenes in the (super) conducting phase was not then known, nor was it known whether the structure was based on the... 

Figure 1.41. Projected equatorial structure (perpendicular to the c-axis) for (a) nominal p4mm symmetry of stage-1 alkali-metal-doped polymers (b) stage-1 alkali-metal-doped polyacetylene (c) stage-1 Cs-doped PPV (Reproduced from ref 104 with kind permission. Copyright (1992) American Physical Society.)...

Addition of methanol to 2-methylene-1,3-dioxepane (28) leads to the corresponding seven-membered cyclic orthoacetate (156) (Equation (23)) <86TL1587>, and electrochemical reduction of 1,3-dioxepan-2-one (157) in an electrolyte containing alkali metal ions affords orthoester derivatives (158), useful for stabilizing /i-doped polyacetylene as the anode-active material of a battery (Equation (24)) <85JAP(K)60I2628I>. 

The partially reduced polyacetylene films formed are gold and have become good electrical conductors (16). It should be noted that in addition to the heavier alkaline earth metals, all the alkali metals (Li, Na, K, Rb and Cs) dissolve in liquid ammonia and they can be incorporated into polyacetylene as n-dopants using this method. An earlier preliminary report to our work had suggested the use of sodium-ammonia solutions for n-doping polyacetylene (17). 

FIGURE 5.8. Structures of some common doped conductive polymers, each projected on a plane normal to the direction of the polymer chains. Cross-hatched circles indicate dopant ions, (a) The square channel structure of potassium-doped polyacetylene (other alkali-doped PA lattices show similar structures),(b) lithium-doped polyparaphenylene, (c) potassium-doped ppp,(ii3) AsFs-doped PPR In (d) the dopant species, represented by x, are probably... 

Harris, A.B. 1994. Mean-field theory for alkali-metal-doped polyacetylene. Phys Rev B 50 12441. 

Long chains without any specified number of carbon atoms are referred to as a polyacetylene polymer. Oxidative (I2) or reductive (alkali) doping leads to conductivity. During the 1970s, Japanese chemists, including H. Shirakawa, were able to polymerize PA (-CH=CH)x-CH=CH2. In 1978, H. Shirakawa, A. G. MacDiarmid, and A. J. Heeger added I2 as a dopant and found a high conductivity. 

Morton-Blake, D. A., An electrostatic investigation of alkali-metal-doped polyacetylene, Synth. Met., 35, 281-293 (1990). 

Acceptor-doped complexes of polyacetylene typically have low thermal stability.However, Delannoy et al. have found that alkali-metal complexes of polyacetylene have surprisingly high thermal stability. More specifically, potassium-doped polyacetylene maintains high conductivity and does not undergo measureable decrease in structural perfection after a 16 hour anneal in inert atmosphere at 200 C. In fact, the potassium-doped cis-polyacetylene increases electrical conductivity by about a factor of five during high temperature thermal annealing and... 

Sen and Chakrabarti have employed the CPHF, CPKS/B3LYP, TDDFT/ALDA, and TDCDFT/VK (time-dependent current density functional theory/Vignale Kohn) methods to calculate the polarizability in alkali-doped traws -polyacetylene chains as a function of chain length and as a function of the nature of the dopant. They have evidenced a minimum in the evolution of the average polarizability per unit cell and have attributed it to the charge transfer between the alkali and the chain. 

Figure 5, Raman spectra at room temperature induced by alkali-metal doped polyacetylene for.XL 676,4 nm...

Recently a great deal of interest has been shown in polyacetylene films that can incorporate alkali metal ions reversibly at a cathode to form electronically conducting compounds of the form (CHNayXr- An all-polymer solid-state battery has been developed in which the electrolyte is a sodium iodide-polyethylene oxide and the electrodes are doped polyacetylenes ... 

The conductivity of polyacetylene is also increased by dopants that are electron donors. For example, the polymer can be doped with alkali metals to give, for example, [Li5 (CH) ln. The wide range of conductivities produced by these two forms of doping is illustrated in Figure 6.3. 

Doping of polyacetylene has been studied extensively, especially n-type doping with alkali metals and p-type doping with iodine. Since 1990, however, papers on this topic have almost ceased to appear, presumably because interest has shifted to other polymers and other properties (fluorescence for instance, which is not shown by polyacetylene). The number of papers that appeared in the 1980 s is so large as to make it impracticable to discuss the contents of each. The development of the current knowledge will be sketched and relevant contributions to the field will be mentioned where this is appropriate. 

Reduction can be effected by doping with alkali metals, either in the vapour phase or in solution, e.g. with naphthalide salts in THE Electrochemical doping is very well possible, for instance with LiB(Me)4 or NaB(Ph)4 in solution with a polyacetylene cathode. The representation (CH)M is often used to denote a certain composition. 

The formulas given in Table 1 are based solely on weight uptake data after exposure of the polyacetylene films to metal-ammonia solutions followed by washing and vacuum drying. To determine the actual metal content of the doped polymers atomic absorption measurements were carried out. These show conclusively that ammonia molecules are coinserted in the polyacetylene films along with the metal cations. This can be compared to the coinsertion of the solvent tetrahydrofuran which has been found to coordinate with the alkali metal ions Li and Na (22,23). The sodium doped samples coinsert slightly more than about one ammonia molecule for each sodium ion. The real formula for the sodium... 

Baughman, R.H., L.W. Shacklette, N.S. Murthy, G.G. Miller, and R.L. Elsenbaumer. 1985. The evolution of structure during the alkali metal doping of polyacetylene and poly(p-phenylene). Mol Cryst Liq Crys 118 253. 

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